40 THE MEDALS OF CREATION. Cyap. III. 
lime, held in solution by an excess of carbonic acid, not in 
actual combination with the lime, but contained in the water, 
and acting as a menstruum, is commonly found in all waters. 
An absorption of carbonic acid, or a loss of that portion 
which exists in excess, will therefore occasion the lime to be 
set free, and precipitated on the foreign bodies in the water, 
as stones, twigs, leaves, &e. 
The substance thus deposited is termed tufa, or traver- 
tine ;* and in some parts of Italy, and of our own Island, 
it constitutes beds of stone of great extent, in which bones, 
shells, and the impressions of leaves and stems, are preserved. 
The stalactites and stalagmites of caverns have a similar 
origin ; many of these caves are of incalculable antiquity, 
and beneath their stalagmitic floors, the bones and teeth of 
extinct carnivorous animals are found in vast quantities. 
SILICIFICATION, or petrifaction by Silex or Flint.—Silex, 
or the earth of flint, is held in solution in large proportions, 
in certain thermal or boiling springs, which, on cooling, 
deposit the siliceous matter (in the same manner as the 
travertine is precipitated from incrusting streams) on foreign 
substances, and produce exquisite chalcedonic infiltrations 
of mosses, &c. But this operation is now only known to be 
in activity in the immediate neighbourhood of foci of vol- 
canic action, as in the celebrated Geysers of Iceland ( Wond. 
p. 95), and the boiling springs of the volcano of Tongariro, 
in New Zealand (98). We have everywhere evidence that 
in former periods, the petrifaction, as well as the incrus- 
tation of organic bodies by silex, was carried on to an 
immense extent; and, doubtless, far beneath the surface, 
the same operation is at the present moment in constant 
progress, and effecting as important changes in the consoli- 
dation of loose materials, as in the earlier geological epochs. 
* Travertine, so called from the river Tibur, whose waters are loaded 
with calcareous earth—Tiburtina, Ital. travertina. 
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